As an information output apparatus in, for example, a wordprocessor, personal computer, or facsimile apparatus, a printer which prints desired information such as characters and images on a sheet-like printing medium such as a paper sheet and film is widely used.
As a printing system for a printer, various schemes have been known. In recent years, inkjet systems have received a great deal of attention because information can be printed on a printing medium such as a paper sheet in a non-contact manner, color information can be easily printed, and printing is very quiet. Of such inkjet systems, a serial printing arrangement of performing printing while reciprocally scanning a printhead, which discharges ink in accordance with desired printing information, in a direction crossing the conveyance direction of a printing medium such as a paper sheet has been widely used because of low cost, small size, and the like.
The basic operation of such an inkjet printing apparatus will be described below. First of all, printing media are fed one by one from a paper feed unit, on which printing media are stacked, by a feed roller. The fed printing medium is repeatedly conveyed by a predetermined amount by a roller pair comprising a conveyance roller and a pinch roller. With regard to the scanning direction, a carriage on which a printhead is mounted is moved (scanned) in a direction almost perpendicular to the above conveyance direction by a carriage motor to place the printhead at a target image formation position.
The positioned printhead discharges ink onto the printing medium in accordance with a signal from an electric board. Image formation on the printing medium is performed by alternately repeating main scanning operation and sub-scanning operation. In the main scanning operation, the carriage is scanned while printing is performed by the printhead. In the sub-scanning operation, the printing medium is conveyed by the conveyance roller.
It is however known that the conveyance amount of a printing medium by the conveyance roller sometimes differs from a target predetermined conveyance amount (set value) due to, for example, a change in the friction coefficient between the conveyance roller and the printing medium or the fictional force which the printing medium receives from the conveyance path.
FIGS. 11A to 11E are schematic views showing the pattern of printed dots formed on a printing medium when the conveyance amount of the printing medium by the conveyance roller coincides with a set value. FIGS. 11A to 11E show a case wherein printing in a predetermined area is completed by making a printhead having a predetermined number of nozzles repeat main scanning four times, that is, so-called 4-pass printing is performed.
Referring to FIGS. 11A to 11E, each circle “◯” represents a printed dot, and the number inside the circle indicates that the dot was printed by scanning operation in a specific ordinal position. FIG. 11A shows the pattern of printed dots formed by the first main scanning operation. When the first main scanning operation is complete, sub-scanning is performed to convey the printing medium by a predetermined amount. FIG. 11B shows the pattern of printed dots formed by the second main scanning operation. Subsequently, main scanning and sub-scanning are repeated in the order shown in FIGS. 11C, 11D, and 11E to complete printing in an 8×8 dot area.
FIGS. 12A to 12E are views showing the pattern of printed dots formed on a printing medium in the same manner shown in FIGS. 11A to 11E when the conveyance amount of the printing medium by the conveyance roller becomes smaller than the set value. After the first scanning operation shown in FIG. 12A, printed dots are formed at the positions shown in FIG. 12B because the conveyance amount is slightly smaller than the set value. Subsequently, since the conveyance amount in each sub-scanning operation is smaller than the set value, printing of a dot pattern in an 8×8 dot area is finally complete as shown in FIG. 12E.
Since the conveyance amount in each sub-scanning operation is smaller than the set value, the dot pattern becomes relatively dense in the conveyance direction. As a consequence, a portion with a noticeable overlap of printed dots appears at the position indicated by the arrow in FIG. 12E. This overlap of dots becomes a black stripe which periodically appears on a completed image. This black stripe becomes more noticeable as the conveyance amount of the printing medium becomes smaller than the set value, resulting in a deterioration in image quality.
FIGS. 13A to 13E are views showing the pattern of printed dots formed on a printing medium in the same manner shown in FIGS. 11A to 11E when the conveyance amount of the printing medium by the conveyance roller becomes larger than the set value. After the first scanning operation shown in FIG. 13A, printed dots are formed at the positions shown in FIG. 13B because the conveyance amount is slightly larger than the set value. Subsequently, since the conveyance amount in each sub-scanning operation is larger than the set value, printing of a dot pattern in an 8×8 dot area is finally complete as shown in FIG. 13E.
Since the conveyance amount in each sub-scanning operation is larger than the set value, the dot pattern becomes relatively sparse in the conveyance direction. As a consequence, a noticeable non-dot portion appears at the position indicated by the arrow in FIG. 13E. This non-dot portion becomes a white stripe which periodically appears on a completed image. This white stripe becomes more noticeable as the conveyance amount of the printing medium becomes larger than the set value, resulting in a deterioration in image quality.
Japanese Patent Laid-Open Nos. 06-238969 and 07-314788 disclose a technique of correcting the conveyance amount of a printing medium in order to prevent a deterioration in printing image quality due to conveyance amount errors like those described above.
It has become clear that even if the conveyance amount of a printing medium in the above case is corrected, the following problem exists.
Even if a printing medium is conveyed by a conveyance amount made to coincide with a set value by correction, stripes may occur on a printed image. FIG. 14 is a schematic view showing the trajectories of ink droplets discharged from a printhead by using arrows. As shown in FIG. 14, the ink droplets discharged from the respective nozzles of the printhead are influenced until landing on the printing medium by the airflow produced when the printhead moves in the main scanning direction or the airflow produced when ink droplets move in mass in the air.
For this reason, the ink discharged from the end portions of the nozzle array is deflected in a direction toward the middle of the nozzle array. As a consequence, when printing is performed with the conveyance amount set on the assumption that ink droplets discharged from the printhead are landed immediately below the nozzles, white stripes occur on the image. Note that the influence of such an airflow varies depending on the number of nozzles used for printing and the number of print passes. The reason why the influence varies depending on the number of print passes is that the density of ink discharged in one main scanning operation varies depending on the number of print passes.
When printing is to be performed while the conveyance amount is changed in accordance with the position of a printing medium, correction can be performed in accordance with the friction coefficient between the conveyance roller and the printing medium and the frictional force which the printing medium receives from the convey path by setting correction amounts corresponding to the respective different conveyance amounts to the values obtained by multiplying the respective conveyance amounts by predetermined coefficients.
When, however, the respective conveyance amounts are to be corrected considering the influence of the above airflow as well, it is not sufficient to only multiply the respective conveyance amounts by the predetermined coefficients. That is, the image quality of a printed image deteriorates.